Chromium plating



Patented Dec. 13, 1932 ELMEB J. WALTZ, OF RIVERSIDE, NEW JERSEY CHROKIUM PLA'IING Ho Drawing.

My invention relates to electrolytic deposition of chromium and more particularly to a process of electroplating with chromium and a bath or aqueous solutlon for usein connection there-wlth.

One object of my invention is to provide, in

a process for producing chromium plating, a bath characterized by its improved throwing power." A further object is to produce chromium plating of bright lustre, even density and lasting quality.

I have found that abath containing chromic acid in water solution can be made. to possess improved throwing power for chromium plating when there is liberated in the bath hydrocyanic acid (HON) or cyanide (CN) ions. I therefore add to a bath containing chromic acid (CrO in excess, an

- alkaline cyanide, as for example sodium cyanide (NaGN), which reacts to liberate hydrocyanic acid (HCN), the other product of the reaction being sodium dichromate (Na Cr O The presence of alkaline chromates or bichromates in the chromic acid solutionalso improves the throwing power. Chromic acid in solution is only slightly ionized whereas alkaline chm-mates and bichromates are strongly ionized. Accordingly the salts resulting from the reaction produce better electrical conductivity in the bath, by virtue of the larger number of dissociate gCrO ions therein, and freer deposition 0 chromium is-permitted.

The followin constitutes the proportions of chromic aci and sodium cyanide which give the most satisfactory results:

Chromic acid 18oz. per gallon Sodium cyanide 1 oz. per gallon Water 1 gallon With this electrolyte I have fOlllld that low carbon steel anodes give excellent results. I.

Application filed January 21, 1929. Serial 30.334431.

then laced in an alkali cleaner. When cleaned the object is rinsed in cold water and scrubbed to remove the cleaner film. I then preferably subject the object to successive baths of nickel, copper and nickel, though plating may be effected directly on copper, brass, steel or nickel. It is not necessary that the work be chemically clean preparatory to plating, but it is importantthat it have a so highly polished surface.

The bath is prepared in accordance with the above formula and maintained at a comparatively coldtemperature for best results. Good plating is obtained with'an electric current of from 3 to 6 volts and a current density in the neighborhood of amperes per square foot. Under the above conditions a serviceable plate with bright lustre and uniform density may be obtained in from 1 to 2 minutes. From time to'time the electrolyte may be replenished by the addition of chromic acid, care being taken to avoid excess acid as dark tones will be caused thereby. If thebath should lack throwing power, so-

dium cyanide should be added in small por- 'tions until. the solution contains its normal proportions. It will be readily understood that I do not limit myself to the precise details of procedure described above and that-the ratio of alkaline cyanide and chromic acid per gallon of solut1on used in the electrolyte may be modified within reasonable limits without departing from the spirit of my invention.

Having thus described my invention, I

claim:

A bath for electrodepositing chromium containin per ga lon and sodium cyanide 1 ounces per gallon.

In testimony whereof, I have hereunto signed my name at Philadelphia, Pennsyl: vania, this 17th day of January, 1929. Y

. ELMER J. WALTZ.

a solution of chromic acid 18 ounces 

